Shrinking a cast/mold. Can it be done?

A bit of a question about the above.

I've unfortunately made my recent helmet a bit too big for myself (very tiny noggin!) and I will more than likely just make the sculpt smaller if I have to but just wondering has anyone shrunk their cast before?

I know recasts loose a good but of size but would that would involve alot of materials to cast, mold, cast mold etc until it shrinks.

Is there any resins out there that are very bad for shrinkage or would I be best lest say making a latex cast and foam filling it then remolding? (as the latex would def shrink)

Cheers!

There are some water based urethanes from Industrial Polymers that are made specifically for shrinking/enlarging, but you might be better off having it scanned and carved or printed- it's much more effective to adjust size as you can scale the 3d file.

I have heard friends making an alginate mold and letting it shrink a bit then running a clay press.

Alginate will shrink, it will also deform in shape the more moisture it loses. But it can work nicely if you clay press and do some re-sculpting. Cheap too.

Not helpful, but related.

Moulding plastic which expands in proportion when exposed to water.

https://www.inventables.com/technolo...anding-plastic

Just thinking out loud here...what about wax?? I'm sure I read somewhere that the wax work sculptures are sculpted so many percent bigger due to shrinkage???

Leigh said:

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Just thinking out loud here...what about wax?? I'm sure I read somewhere that the wax work sculptures are sculpted so many percent bigger due to shrinkage???

I'm sure that's meant as the molding and casting process results in shrinkage of the casts made.

I have used Hydrospan 400 and it worked great for shrinking. They say to do about a 40% ratio and you will get 40% shrinkage (that's from memory so don't quote me on it)

I've successfully shrunk castings by 40% using it. I'm sure adjusting the water / resin ratio would give different results.

It takes a few days, and leaves a full proportioned brownish rubber part.

Maybe get a sample and do some testing

Lawrence

Thanks for the tips so far everyone.

I was thinking alginate aswell but doing a solid pour into the mold and letting the cast shrink but ive heard it can be a pain to dry and may warp.

Thanks for the info on that Hydrospan Lawrence. I had a look at their site and it states that the largest it can be cast is 59% of the original size. That may be too small but no harm in giving it a shot

Keep the suggestions coming!

One more question for you Lawerence,

What size castings did you shrink and were they hollow or solid? The reason I ask is the only place I can find to buy is online and over in the US plus its pricey, 40USD for a quart (just about enough for a hollow cast of the helmet) so if the hollow casts dont hold up then I may have to just resculpt or do the old mixing the silicone with thinner and remold trick.

Thanks!

The castings I made were solid and about 6" x 4" so you would be working with a larger part than I did. They were also cast solid so I don't know the limitations of a hollow casting. I think hollow casting might be difficult, from memory there was almost no gel time and seemed to cure pretty fast.

I will PM you the contact details for my supplier. I am in the UK, and I imagine it would be cheaper to ship to you from here rather than from the US.

I bought mine in March of last year and I think it was £16 for a quart.

Lawrence

About what percentage would you say you need it to shrink by?

Quote:
Originally Posted by Leigh
Just thinking out loud here...what about wax?? I'm sure I read somewhere that the wax work sculptures are sculpted so many percent bigger due to shrinkage???

I'm sure that's meant as the molding and casting process results in shrinkage of the casts made.

No, Leigh is the correct one here. Wax does indeed have a very significant shrink rate. Anything that has to be heated to melt will by definition expand and then shrink as it cools.

Yes, casting in wax will give you a smaller part.

At the foundry they quote 2% shrinkage going from clay to wax.

I think most toy waxes are the same percentage going from clay to wax after a waste mold is done.

There is a slight tendency for the wax to shrink more noticeably from side to side versus top to bottom. When sculptors do the clays for wax, they tend to make them a little more full in the face to counter this.

If you've ever seen wax demolded, you would see that the cooled part has pulled away from the mold slightly from the wax shrinkage.
It is not from the molding process although some use alginate for the waste mold.

Making an alginate mold of the helmet and letting it dry out a bit and then casting a part should work as well. The key is to try and keep your alginate mold as uniform in thickness as you can.

Good luck!!!
Chuck...

@ Lawerence : Thanks for the tips and contact details, no reply from them yet but will keep at them.
@ Endoskeleton : Between 10-20% ideally
@ Leigh, Ozymandius, Clayslinger : Thanks lads, the wax might be worth a try. Any specific type that shrinks or is used for casting?

If all else fails and I dont have time to resculpt it and the wax doesnt provide enough shrinkage I was thinking maybe doing the shrinking mold idea using thinner in the silicone

Mold Making and Casting: Cold Casting resin with metal powders - YouTube

It's mostly a cold casting tutorial, but the start goes over using Shrink It to shrink a skull down by 1/4. Shrink size is determined by the amount of water added.